The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known portable power tools typically have an electric motor received within a tool housing. One common type of electric motor used in power tools has a rotor, a stator, and brushes. The rotor includes a rotor shaft, laminations mounted on the rotor shaft, armature windings wound in slots in the lamination stack, and a commutator mounted on the rotor shaft and electrically connected to the armature windings. The stator may have field windings wound in laminations, or may have permanent magnets. The brushes are mounted in brush housings, often known as brush boxes or brush holders, in sliding electrical contact with the commutator. Electric current is supplied from a power source through the brushes to the commutator, and from the commutator to the armature windings.
The brushes and brush holders are typically part of a brush assembly(ies). The brush holders and brushes are disposed diametrically opposite to each other with the commutator disposed therebetween. The brush assembly(ies) includes springs that urge the brushes against the commutator. Exemplary brush assemblies may utilize two or four brushes around the commutator.
Generally, in brush assemblies, transmission of electric current across the sliding interface between the brushes and the motor commutator generates a considerable amount of heat. This heat is often associated with the material used in the brushes. Such materials have relatively higher electrical resistance in order to enhance desirable brush properties such as low friction coefficient and high durability. The heat is also generated from the electrical arcing that occurs when electrical switching occurs as the commutator and brushes rotate with respect to each other. It is desirable for this heat to be transferred away from the brushes and dissipated, for example by impinging air flow.
Conventional brush holder designs often include a brush card mount that is made of a plastic material and the brush holders mounted on a top surface of the brush card mount. The plastic brush card mount separates the brush holders and the brushes from the rest of the motor, where a cooling fan is usually disposed to cool the motor stator and armature. This results in a high thermal coefficient between the brushes and the cooling air, which correspondingly results in high temperatures. Some conventional designs utilize larger brushes or larger air flow paths to increase the surface area of the brushes or brush holders in the path of the airflow, but such designs result in larger and more expensive motors.
Another disadvantage of above-described conventional designs is the mechanical mounting and assembly of the brush holders over the brush card mount. In these designs, the brush holders are mounted on the top surface of the brush assembly, and crimp connections are generally used to secure the brush holders to the brush card mount. These crimp connections provide added tolerances that add to the overall size of the motor assembly. In particular, in designs where a motor fan is provided adjacent the brush assembly, the distance between the fan and the brush assembly must be increased to account for the added tolerances of the crimp connections.
What is needed is a brush assembly design that enables more efficient heat transfer from the brushes and brush holders. Also needed is a brush assembly design that reduces the tolerances associated with the conventional brush holder mounting methods.